I block stream del Massiccio Peridotitico di Lanzo (Alpi Nord-Occidentali)

The Lanzo Peridotite Massif (LPM) is one of the largest outcrops of ophiolitic peridotites in the world and it is well known because of the freshness of its mantle rocks; furthermore, the LPM represents a peculiarity also for the extraordinary and unusual presence of a large number of block streams (157), consisting of streams of angular blocks resting over a thick succession of fine sediments. The LPM extends for about 150 km2 along the internal margin of the Western Alps, 35 km North-West of Turin, between the lower Susa Valley and the lower Viù Valley; it is commonly divided into a southern body (55 km2), a central body (90 km2) and a northern body (5 km2), which are separated by two broad mylonitic and serpentinized shear zones with a NW-SE direction. Other tectonic shear zones are localized in the marginal and more serpentinized parts of the LPM, corresponding to the contacts with the adjacent tectonic units of the alpine chain (i.e. Piemontese Zone and Sesia-Lanzo Zone). The LPM is mainly composed of massive weakly serpentinized plagioclase peridotites with minor spinel peridotites and granular harzburgites. Plagioclase peridotites are well exposed in the core of the three bodies, whereas in the peripherical areas peridotites gradually become more serpentinized till turning into massive serpentinites. Spinel lherzolites, spinel dunites and harzburgites are subordinated; all the peridotites are frequently intruded by various gabbroic veins and dykes and sometimes by basalts. Block streams mainly extend on lherzolite outcropping areas, covering on the whole 14.67 km2, equivalent to 9.8% of the LPM extent. Block streams are located chiefly on the northern and central body of the LPM, along the slopes of the Mount Colombano - Mount Corno and Mount Colombano - Mount Lera ridges; another notable group of block streams is localized between Col del Lys and Colle Portia, whereas in the southern areas of the LPM such features are rather scattered. Block streams range in elevation between 1645 and 470 m a.s.l., with a maximum relief of 870 m; they take up an area ranging from 0.3 to 91.6 ha, with a maximum length of 2.23 km and a mean slope of 13÷44°; nevertheless 76% of block streams have a mean slope of 23÷32°. Block streams show an upper layer made of interlocked coarse angular to subangular blocks with no fine matrix, resulting in an openwork framework. Blocky cover has a relatively uniform thickness ranging between 1 and 7 m, commonly with a coarsening-upward texture. Detrital elements range between 0.2 and 1.8 m in diameter and derive exclusively from rocks belonging to the LPM. The upper Il Quaternario Italian Journal of Quaternary Sciences 22(1), 2009 - 3-22 4 G. Fioraso & G. Spagnolo 1. INTRODUZIONE Il Massiccio Peridotitico di Lanzo (MPL), ubicato nel settore interno dell'arco alpino occidentale, oltre a costituire una delle più estese masse di peridotiti di mantello esistenti al mondo (~150 km2) (BOUDIER, 1978), ospita un'eccezionale concentrazione di block stream che coprono complessivamente una superficie di 14,67 km2, pari al 9,8% dell'area di affioramento del MPL. I block stream sono costituiti in superficie da un complesso detritico a grossi blocchi, privo di matrice, poggiante su un potente materasso di sedimenti sabbioso-siltosoargillosi (complesso inferiore); il più delle volte i block stream danno luogo a lingue che in alcuni casi giungono con la propria fronte a sfiorare gli attuali fondovalle. Lo sviluppo di questi fenomeni è spazialmente vincolato dalla presenza di un substrato costituito da peridotiti non metamorfosate o solo parzialmente serpentinizzate. Nell'arco alpino e appenninico non sono riscontrabili altri esempi di block stream confrontabili, per numero, dimensioni e morfologia, con quelli prese